1. Field of the Invention
The present invention relates to photolithography which is applied for precise processing when semiconductor devices are produced, and more particularly relates to an optical exposure method used for photolithography.
When semiconductor devices such as ultra-LSIs are highly integrated and precise processing is required, manufacturers greatly rely on improvements in lithographic technology. Photolithography using light is suitable for mass production. Therefore, it is adopted for economical reasons.
2. Description of the Related Art
In order to improve resolution in optical exposure technology, it is important to increase the numerical aperture (NA) and to reduce the wavelength of light generated by a light source. On the other hand, the focal depth is reduced as NA is increased. Recently, attention has been given to a deformation illumination method (an oblique incident illumination method) which improves the critical resolution and focal depth (for example, shown in pages 28-37 of xe2x80x9cNikkei Micro Devicexe2x80x9d No. 82, April, 1992).
For holes in the diaphragm (the apertures) in the deformation illumination method, zonal holes and four holes symmetrical with respect to a point are well known. In a conventional illumination method, a ray of illumination light sent from a circular hole, coinciding with an optical axis, to a photomask (reticule) is vertically incident and an image is formed by three beams of light of 0, +1, and xe2x88x921. However, with this deformation illumination method, the position of the diaphragm is shifted from the optical axis, so that illumination light sent from the hole is obliquely incident on the photomask, and image formation is conducted by two beams of light of 0 and +1 sent from the photomask. In a focal position, higher contrast can be provided by the conventional illumination method, however, in a defocal position, higher contrast can be provided by the deformation illumination method, so that the focal depth and resolution can be considerably improved.
In the conventional deformation illumination method, i.e., only for a simple line and space pattern, a pattern of the photomask is projected and exposed on a register with a diaphragm having the aforementioned general type of diaphragm holes. Accordingly, the illumination system does not meet the requirement of each pattern, so that the effect of oblique incidence of the deformation illumination method is not sufficient.
Also, recently, attention has been given a lithographic technology using a phase shift mask, and the following pattern forming method has been reported to be an effective technology: an unexposed portion (pattern) is used that is accompanied by a sharp decrease of optical intensity generated by a step portion (the phase of exposure light is changed by 180xc2x0 by this step portion) of a phase shifter of a phase shift mask.
However, when a pattern is formed by this technology, the unexposed portion (pattern) is formed in all step portions of the phase shifter. Therefore, in many applicable fields, it is necessary to provide a process to inhibit the formation of a pattern generated by the unexposed portion generated by an unnecessary step portion of the phase shifter.
Therefore, the following techniques have been conventionally proposed to ease the sharp decrease of optical intensity: another exposure mask is put on the unnecessary unexposed portion so as to conduct an exposure operation (double exposure); and a multi-shifter (step of 90xc2x0) is provided stepwise in a step portion of the phase shifter, the pattern formation of which is not necessary.
However, in the double exposure method that has been conventionally proposed as a method to remove an unnecessary unexposed portion, it is necessary to manufacture a plurality of masks so as to conduct multi-exposures. Accordingly, it is necessary to increase the number of the mask manufacturing processes. On the other hand, it is also necessary to ensure an alignment of the double exposure, so that the throughput is lowered.
Moreover, when a multi-shifter is manufactured, a complicated and difficult process technique is required in order to provide an optically accurate multi-shifter, and further a big problem is caused when a manufactured phase shift is inspected and corrected.
In order to meet the demand of forming minute patterns, for example, attention is given to an oblique incidence illumination method disclosed in the official gazette of Japanese Unexamined Patent Publication No. 2-142111 (1990). According to this method, a ray of light that is vertically incident on a lens is incident being oblique at a predetermined angle, so that focusing is conducted using interference of light.
However, in the aforementioned conventional method, the-same light source is used for any device patterns without giving attention to the profile of the light source. Accordingly, problems are caused.
It is an object of the present invention to provide a projection exposure method with a deformation illumination system optimal for a device pattern (photomask pattern).
It is another object of the present invention to provide a method by which an unnecessary unexposed portion can be easily removed without using the aforementioned multi-exposure method or relying on the technique in which the phase shift mask having a multi-shifter is used, in the case where a pattern is formed using the unexposed portion accompanied by a sharp decrease of optical intensity caused by a step portion of the phase shifter.
It is another object of the present invention to provide an optimization method for a light source profile to obtain an optimal light source profile in accordance with a device pattern.
It is another object of the present invention to realize an optical projection exposure in which resolving power can be provided that is higher than that of the conventional phase shift mask or oblique incidence illumination.
The aforementioned object can be accomplished by an optical exposure method by which a pattern on a photomask is projected and exposed on a register on a base plate with an exposure device including a deformation illumination system composed of a light source, a diaphragm and a condenser lens, and also including a photomask and a projection lens, wherein the optical exposure method uses a ray of linear light for illumination that is parallel with a photomask pattern in a position separate from an optical axis of the exposure device (or the optical exposure method uses two rays of linear light for illumination that are symmetrical with respect to the optical axis) when the photomask pattern is a line and space pattern.
The aforementioned object can be accomplished by an optical exposure method, wherein the optical exposure method uses a ray of first linear light for illumination that is parallel with a first pattern portion in a position separate from the optical axis of the exposure system (or the optical exposure method uses two rays of first linear light for illumination that are parallel with the first pattern portion symmetrical with respect to the optical axis), the optical exposure method also uses a ray of second linear light for illumination that is parallel with a second pattern portion in a position separate from the optical axis of the exposure system (or the optical exposure method also uses two rays of second linear light for illumination that are parallel with the second pattern, two rays of second linear light being symmetrical with respect to the optical axis), when the first pattern portion of line and space, and the second pattern portion of similar line and space make a right angle with each other in the photomask pattern.
Moreover, it is preferable to use an optical exposure method in which the first linear light and the second linear light are oblique by an angle xcfx86 with respect to the optical axis in a position on the photomask, and an equation 2pxc2x7sin xcfx86=xcex is satisfied (where p is a setting pitch of the line and space pattern on the projection surface, and xcex is a wavelength of light).
The object of the present invention can be accomplished by an optical exposure method by which a pattern on a photomask is projected and exposed on a register on a base plate with an exposure device including a deformation illumination system composed of a light source, a diaphragm and a condenser lens, and also including a photomask and a projection lens, wherein the optical exposure method uses a ray of first block light for illumination that is parallel with the bottom surface of the triangular wave in a position separate from the optical axis of the exposure device (or the optical exposure method uses two rays of first block light for illumination that are symmetrical with respect to the optical axis and parallel with the bottom surface of the triangular wave), and the optical exposure method also uses a ray of second block light for illumination that is perpendicular to the bottom surface of the triangular wave (or the optical exposure method also uses two rays of second block light for illumination that are symmetrical with respect to the optical axis and perpendicular to the bottom surface of the triangular wave), when the photomask pattern is a line and space pattern of a triangular shape, the bottom angle of which is xcex8.
Moreover, it is preferable to adopt an optical exposure method in which the optical exposure method characterized in that: the first block light is oblique by an angle xcfx86x with respect to the optical axis in a position on the photomask, an equation 2pxc2x7sin xcfx86x=xcex sin xcex8 being satisfied (p is a setting pitch of line and space pattern in a register, and xcex is a wavelength of light); the second block light is oblique by an angle xcfx86y with respect to the optical axis in a position on the photomask, an equation 2pxc2x7sin xcfx86y=xcex cos xcex8 being satisfied; and a ratio of the illumination area of the first block light to that of the second block light is sin xcex8:cos xcex8.
In the optical exposure method of the present invention, the most appropriate illumination light shape and oblique incident angle xcfx86 are set in accordance with each photomask pattern, so that the resolution and focal depth and improved for each pattern. Especially when the pitch of line and space of a photomask pattern is close to 1:1, the optical exposure method of the invention is especially effective. In this connection, a pattern of line and space corresponds to a plurality of linear shading (or transmission) stripes of a photomask that are disposed in parallel at regular intervals on a developed register pattern.
In the aforementioned equation 2pxc2x7sin xcfx86=xcex, wavelength xcex becomes constant for g ray (434 nm), i ray (365 nm) or excimer laser beam (254 nm for KeF excimer laser beam), and incident angle xcfx86 is determined in accordance with pattern pitch width p.
According to another aspect of the present invention, there is provided a projection exposure method of the present invention comprising the steps of: irradiating an exposure mask with exposing light having an optical intensity distribution extending in a primary direction in its section; and projecting the light transmitted through the exposure mask on a surface to be exposed, wherein exposure is carried out with exposure characteristics relying on a direction of the mask pattern of the exposure mask.
Moreover, the present invention is to provide a projection exposure method comprising the steps of: irradiating a phase shift exposure mask with exposing light having an optical intensity distribution extending in a primary direction in its section; and projecting the light transmitted through the phase shift exposure mask on a surface to be exposed, wherein exposure is carried out with exposure characteristics relying on a direction of the mask pattern of the phase shift exposure mask. Especially, an exposing process is adopted in which exposure is carried out with non-symmetrical exposure characteristics including one direction of a step in which an unexposed portion is formed having a sharp decrease of optical intensity caused close to an edge portion of the phase shifter of the phase shift exposure mask, and also including the otter direction of a step in which an unexposed portion is not formed.